Circuits in motor drives, appliance controls, robotics, lighting ballasts and other applications often require semiconductor switching devices that can carry large currents and support high blocking voltages. The bipolar junction transistor (“BJT”) has been the switching device of choice for many high power applications because of its ability to handle relatively large output currents and support relatively high blocking voltages.
As is well known to those of skill in the art, a BJT is a three-terminal device constructed of a doped semiconductor material. A BJT includes two p-n junctions that are formed in close proximity to each other in a semiconductor material. In operation, charge carriers enter a first region of the semiconductor material (which is called the emitter) that is adjacent one of the p-n junctions. Most of the charge carriers exit the device from a second region of the semiconductor material (which is called the collector) that is adjacent the other p-n junction. The collector and emitter are formed in regions of the semiconductor material that have the same conductivity type. A third region of the semiconductor material, known as the base, is positioned between the collector and the emitter and has a conductivity type that is opposite the conductivity type of the collector and the emitter. Thus, the two p-n junctions of the BJT are formed where the collector meets the base and where the base meets the emitter. By flowing a small current through the base of a BJT, a proportionally larger current passes from the emitter to the collector.
A BJT may be a “PNP” device or an “NPN” device. In a PNP BJT, the emitter and collector are formed in p-type regions of the semiconductor material, and the base is formed in an n-type region of the semiconductor that is interposed between the two p-type regions. In an NPN BJT, the emitter and collector are formed in n-type regions of the semiconductor material, and the base is formed in an p-type region of the semiconductor that is interposed between the two n-type regions.
BJTs are current controlled devices in that a BJT is turned “on” (i.e., it is biased so that current flows from the emitter to the collector) by flowing a current through the base of the transistor. For example, in an NPN BJT, the transistor is typically turned on by applying a positive voltage to the base to forward bias the base-emitter p-n junction. When the device is biased in this manner, holes flow into the base of the transistor where they are injected into the emitter. The holes are referred to as “majority carriers” because the base is a p-type region, and holes are the normal charge carriers in such a region. At the same time, electrons are injected from the emitter into the base, where they diffuse toward the collector. These electrons are referred to as “minority carriers” because electrons are not the normal charge carrier in the p-type base region.
The base of the device is formed to be a relatively thin region in order to minimize the percentage of the minority carriers (i.e., the electrons injected into the base from the emitter) that recombine with the holes that flow from the base into the emitter. The collector-base p-n junction is reverse biased by applying a positive voltage to the collector. This facilitates sweeping the electrons that are injected from the emitter into the base to the collector. The device is referred to as a “bipolar” device because the emitter-collector current includes both electron and hole current. The current that flows into the base of the device controls the emitter-collector current.
Typically, a BJT may require a relatively large base current (e.g., one fifth to one tenth of the collector current) to maintain the device in its “on” state. As high power BJTs have large collector currents, they also typically have significant base current demands. Relatively complex external drive circuits may be required to supply the relatively large base currents that can be required by high power BJTs. These drive circuits are used to selectively provide a current to the base of the BJT that switches the transistor between its “on” and “off” states.
Power Metal Oxide Semiconductor Field Effect Transistors (“MOSFET”) are another well known type of semiconductor transistor that may be used as a switching device in high power applications. A power MOSFET may be turned on or off by applying a gate bias voltage to a gate electrode of the device. For example, an n-type MOSFET turns on when a conductive n-type inversion layer is formed in a p-type channel region of the device in response to the application of a positive bias to the gate electrode. This inversion layer electrically connects the n-type source and drain regions and allows for majority carrier conduction therebetween.
The gate electrode of a power MOSFET is separated from the channel region by a thin insulating layer. Because the gate of the MOSFET is insulated from the channel region, minimal gate current is required to maintain the MOSFET in a conductive state or to switch the MOSFET between its on and off states. The gate current is kept small during switching because the gate forms a capacitor with the channel region. Thus, only minimal charging and discharging current (“displacement current”) is required during switching, allowing for less complex gate drive circuitry. Moreover, because current conduction in the MOSFET occurs through majority carrier transport only, the delay associated with the recombination of excess minority carriers that occurs in BJTs is not present in MOSFET devices, allowing for switching speeds that can be orders of magnitude faster than that of BJTs. The drift region of a power MOSFET, however, may exhibit a relatively high on-resistance, which arises from the absence of minority carrier injection. As a result, the operating forward current density of a power MOSFET is typically limited to relatively low values as compared to power BJTs.
Devices embodying a combination of bipolar current conduction with MOS-controlled current flow are also known. One example of such a device is the Insulated Gate Bipolar Transistor (“IGBT”), which is a device that combines the high impedance gate of the power MOSFET with the small on-state conduction losses of the power BJT. Another device that combines MOSFET and BJT is the MOSFET Gated Transistor (“MGT”). An MGT may be implemented, for example, as a Darlington pair of discrete high voltage n-channel MOSFET at the input and a discrete BJT at the output. The MOSFET supplies the base current of the BJT while presenting minimal load to external drive circuits. The MGT may combine the high temperature, high current density switching characteristics of the BJT with the minimal drive requirement of the MOSFET.
Most power semiconductor devices are formed of silicon (“Si”), although a variety of other semiconductor materials have also been used. Silicon carbide (“SiC”) is one of these alternate materials. SiC has potentially advantageous semiconductor characteristics including, for example, a wide band-gap, high electric field breakdown strength, high thermal conductivity, high inching point and high-saturated electron drift velocity. Thus, relative to devices formed in other semiconductor materials such as, for example, Si, electronic devices formed in SiC may have the capability of operating at higher temperatures, at high power densities, at higher speeds, at higher power levels and/or under high radiation densities.